AFM Files & Kerning

Question

I am on a mac snow leopard working mostly in Illustrator. Others apps InDesign, After Effects, Premiere. Can someone please explain AFM files, how and  when to use them?
Please correct or confirm, but my understanding is that AFM information is in the bitmapepd files of older formats, and within opentype, so there really is no need for .AFM files on the mac.
If the kerning information is in the font already, then why do you always have to kern ones and periods in most fonts. Optical seems to do a better job than auto, but I often later have to tighten the tracking. Is auto using the AFM infomration in the font, and optical is using the Illustrator engine to calculate new kern pairs.

Answer 1

AFM (Adobe Font Metric) files contained detailed metrics associated with legacy Type 1 fonts including not only advance widths, but also information such as pair kerning information that would/should be used for layout programs. On the MacOS, most of the AFM information was also included in the bitmap format files that accompanied the actual font outline file. Since the days of Adobe Type Manager and then the built-in Type 1 font renderer in MacOS, the only reason for the bitmap format files was for this font metric information and in fact, you only needed one such bitmap font file per outline file. For Windows, much of AFM information is stored with the .PFM (PostScript Font Metrics) file that accompanies and is installed with the .PFB (PostScript Font Binary) file containing the actual font outlines and some minimal metrics.
TrueType, TrueType OpenType, and OpenType CFF fonts have all font metrics included in the .TTF or .OTF files themselves and as such, do not require any secondary font metrics information.
Whether you kern using the metrics or automatic/optical option, really depends on your tastes and the font itself. The optical option algorithmically creates pair kerning based upon the actaul fit of two glyphs with each other. The techniques are rather sophisticated, but work amazingly well (except when they don't ).
          - Dov